Previously, only one sampling operation could be running and it was
generally used to profile the startup of the browser.  To make it more
useful, it can now run against any thread and multiple profilers can
execute in parallel.

Sampling will continue until the desired number of samples has been
collected, it is manually stopped, or the controlling object gets
destructed.

The SamplingThread is a singleton base::Thread that is self-managing.
- It is started (via GetOrCreateTaskRunnerForAdd) on the calling
  thread when work arrives.
- It stops (via ShutdownTask) on its own thread when it has been
  idle for 1 minute.
- DetachFromSequence is called after both of these to allow for
  accessing the API from different threads.
- thread_execution_state_lock_ is held when doing Thread API calls to
  ensure that access is sequenced.

The sampled thread is expected to live at least as long as the
thread controlling the sampling.

BUG=671716

Review-Url: https://codereview.chromium.org/2554123002
Cr-Commit-Position: refs/heads/master@{#465614}